Recovery operation of a diode is an operation in which the diode transitions from an ON (current-carrying) state to an OFF (current-shutdown) state in accordance with a switching operation of switching elements that form a pair. In such a case, as a potential difference between an anode and a cathode increases, a depletion layer centered on a PN junction spreads inside the device.
In an ON state, a large quantity of carriers (on the order of 100 to 10,000 times an impurity concentration of a substrate) is flowing through the device. Due to an electric field in the depletion layer during the recovery operation, positive holes are attracted to the anode side and electrons are attracted to the cathode side, and these are finally exited from the anode electrode and cathode electrode respectively.
Since no voltage is applied to the diode in the ON state, almost no energy loss occurs even when a high current flows (actually, a voltage corresponding to a current=on the order of several V is applied, which constitutes an ON loss of the diode). On the other hand, during the recovery operation, a current flows with a high voltage applied, a large energy loss and heat are generated. Therefore, the higher the current in the ON state, the more carriers exist inside the device, and a high current flows during the recovery operation, and heat produced thereby may cause a thermal destruction.
There are indices indicating recovery characteristics of a diode such as recovery operation loss, and reverse recovery current. It has been an object of diode development so far to improve these indices. These indices are set to reduce diode losses at the time of normal switching, and improving these indices can also result in improvement of overload tolerance of thermal destruction. If the breakdown voltage is improved, breakdown will not take place even when a current density increases, and it is thereby possible to reduce the chip size and cost.
Moreover, the depletion layer spreads not only in a longitudinal direction of the device between the anode and cathode but also in a plane direction. When the depletion layer which has spread in the plane direction reaches a chip end, the withstand voltage may become unstable and discharge may occur. In order to prevent the depletion layer from reaching the chip end, a termination region (inactive region) is provided which includes neither anode layer nor anode electrode and does not perform device operation, in addition to an active region which includes an anode layer and performs device operation (e.g., see Patent Literature 1).